1. Field of the Invention
The present invention generally relates to an electric motor and, more particularly, to a direct current commutator motor of a type that can be driven by a direct current power source obtained by rectifying an alternating current power source.
2. Description of the Prior Art
The prior art direct current commutator motor that is driven by a direct current power source obtained by rectifying an alternating current power source has a problem in that because a brush electrically connected with a negative pole of the DC power source is susceptible to wear by combustion as a result of collision of cations, no sufficient lifetime can be secured.
To eliminate the above discussed problem, the Japanese Utility Model Publication No. 54-16242, published Jun. 27, 1979, discloses the use of a standard brush for connection with a positive pole of the DC power source and a metal-mixed graphite brush for connection with a negative pole of the DC power source. The metal-mixed graphite brush is prepared by sintering and molding a mixture of graphite with metallic particles dispersed in the graphite. Since the metallic particles mixed in the graphite have a property less susceptible to wear by combustion as a result of collision of cations, the use of the metal-mixed graphite brush for connection with the negative pole of the DC power source is effective to prolong the service life of the brush.
With reference to FIG. 6, the prior art direct current commutator motor will be discussed. Referring to FIG. 6 showing the prior art commutator motor partly in a longitudinal sectional representation and partly in a longitudinal side representation, reference numeral 1 represents a frame having a magnet 2 built therein. Reference numeral 3 represents a bracket made of a thermoplastic resin. The bracket 3 has defined therein an insertion hole 3a for receiving a specials tube 10 and a positioning hole 3b extending transverse to the insertion hole 3a and is fixed immovable relative to the frame.
Reference numeral 4 represents a rotor rotatably supported by bearings carried respectively by the frame 1 and the bracket 3. Reference numeral 7 represents a commutator fixed on a shaft for rotation together with the rotor 4. This commutator 7 is electrically connected with opposite ends of a wire winding of the rotor 4. Reference numeral 8 represents a carbon brush for connection with a positive pole of the direct current power source, which brush 8 is hereinafter referred to as a plus-side brush. This positive brush 8 is slidably accommodated within a plus-side specials tube 10 and is normally biased by a coil spring within the plus-side specials tube 10 so as to slidingly contact the commutator 7.
Reference numeral 9 represents a metal-mixed graphite brush for connection with a negative pole of the DC power source, which brush 9 is hereinafter referred to as a minus-side brush. In a manner similar to the plus-side brush 8, the minus-side brush 9 is slidably accommodated within a minus-side specials tube 10 and is normally biased by a coil spring within the minus-side specials tube 10 so as to slidingly contact the commutator 7. The plus-side and minus-side specials tubes 10 are of an identical shape and structure and have a spring-like cutout 10a formed therein. The spring-like cutout 10a is capable of being deformed as the specials tube is inserted into the insertion hole 3a to press and deform an inner wall surface of the insertion hole 3a to thereby advance deep into the insertion hole 3a until it is brought into engagement with the positioning hole 3b to thereby restrain the specials tube from being separated out of the insertion hole 3a. 
In this direct current motor of the structure described above, when a direct current voltage obtained by full-wave rectifying an alternating current power source is applied to the brushes through the respective specials tubes, the current flows through the wire winding of the rotor by way of the commutator then held in sliding contact with the brushes, developing an electromagnetic force between the wire winding and the magnet 2 to thereby cause the rotor to rotate. At this time, even though the brushes then held in sliding contact with commutator pieces to supply an electric power to the wire winding through the commutator are separated a distance therefrom as a result of rotation of the rotor, the electric current continues to flow therethrough to the wire winding by way of spark discharge developed therebetween, so long as such distance is minute.
The spark discharge occurs on both plus-side and minus-side and constitutes a cause of wear by combustion of graphite forming the brush, which leads to reduction of the service life. Accordingly, occurrence of the spark discharge should be minimized to secure a sufficient service life of the brush.
However, considering that the prior art direct current commutator motor makes use of the direct current power source that is obtained by full-wave rectifying the alternating current power source, and that the electric power source is repeatedly turned on and off at a cycle twice the frequency of the alternating current power source, a problem has been found in that the spark discharge occurs frequently, accompanied by an unstable rectification.
Although to minimize the occurrence of the spark discharge, it is necessary to increase the electric resistance of the brush to a certain extent so that the spark discharge can vanish at a small distance between the brush and the commutator pieces, the metal-mixed graphite brush used as a minus-side brush in the prior art DC commutator motor has an extremely low resistivity, say, that of {fraction (1/100)} to {fraction (1/1,000)} of the resistivity of the standard carbon brush and, therefore, it has been found difficult to increase the electric resistance of the brush.
The reduction in service life of the brush resulting from the above discussed spark discharge is considerable with increase of the rotational speed of the motor, and the increase of the rotational speed of the prior art DC commutator motor is therefore limited. Also, the prior art DC commutator motor has an additional problem in that since the specials tubes employed respectively for the plus-side and the minus-side are of an identical shape there is a high possibility that they are inserted erroneously into the wrong insertion holes.
Furthermore, since in the prior art compact DC motor prevention of the specials tubes from being separated is implemented by the engagement between the cutout 10a and the positioning hole 3b, the inner wall surface of the insertion hole has t be pressed and deformed as the respective specials tube is inserted into the insertion hole and, for this purpose the thermoplastic resin is used as a material for the bracket. Accordingly, as compared with the use of the thermoplastic resin, not only is the reliability with respect to heat low, but a dimensional precision is hardly attained and no sufficient physical strength can be attained.
Accordingly, the present invention has been devised to substantially eliminate the above discussed problems inherent in the prior art direct current commutator motor and is intended to provide an improved direct current commutator motor having a prolonged lifetime and a high reliability and capable of being driven at a high speed.
To this end, the present invention provides a DC commutator motor that is driven by a direct current power source obtained by rectifying an alternating current power source, wherein a plus-side is made of material having a high resistivity and a minus-side brush is made of material having a low resistivity and, at the same time, a capacitor is inserted in and connected parallel to the direct current power source. This design is effective to provide the DC commutator motor having the brushes of long service life with occurrence of the spark discharge minimized and capable of being driven at a high speed.
More specifically, the plus-side and minus-side brushes are preferably made of carbon and metal-mixed graphite, respectively. This is particularly advantageous in that the resistance of the minus-brush to the cations can be increased and, at the same time, the occurrence of the spark discharge can be reduced to minimize the possible wear of the brush.
Plus-side and minus-side specials tubes employed in the practice of the present invention are preferably of a shape differing from each other, so that the possibility of the specials tubes being inserted into the wrong insertion hole can advantageously be avoided. More specifically, the DC commutator motor embodying the present invention makes use of first and second specials tubes made of metal and adapted to slidably receiving respective brushes of different material, and a bracket made of a synthetic resin and having first and second insertion holes defined therein. The first specials tube and the second specials tube can be snugly inserted into the first and second insertion holes, respectively, but the first and second specials tube cannot be inserted into the second and first insertion holes, respectively. Accordingly, there is no possibility that the specials tubes may be erroneously inserted into the wrong insertion hole during assemblage of the commutator motor.
Also, in the practice of the present invention, prevention of the specials tubes from being separated is implemented without relying on the spring property, but by the utilization of deformation of a tab. More specifically, each of the specials tubes employed in the commutator motor embodying the present invention has one side face provided with a tab that is divided into first and second tab segments with a cutout intervening therebetween. On the other hand, the corresponding insertion hole has a groove communicated therewith, said groove being of such a shape that when the specials tube is inserted into the insertion hole, the tab can be accommodated loosely therein with a slight gap and, when the specials tube is inserted to a predetermined position inside the insertion hole, only one of the tap segments is exposed to the outside from the insertion hole and is then deformed to bend. Since the inner wall of the insertion hole need not be elastically deformed during the insertion of the corresponding specials tube, a thermosetting resin can advantageously be used as material for the bracket. Therefore, not only is the reliability with respect to heat high, but also a dimensional precision can be attained and a sufficient physical strength can be secured.